The present invention relates, in general, to structured mass transfer packings.
It is generally known to use so-called structured packings in columns for implementing a mass transfer between two fluid phases. Examples of such columns include columns for absorption, desorption, rectification or extraction. The packing is comprised of packing elements arranged on top of each other in various packing beds zones of the column. The packing elements may be configured as single packing elements that match and substantially fill the column cross section, or, especially when the columns have greater inner width, may be composed of single segments which are then combined to a packing element, e.g., like pieces of a cake.
There are various designs of structured mass transfer packings which can also be made of different materials. Packings are known which are made of glass, e.g., packings made through joining corrugated glass plates, as described in German Pat. Nos. 39 09 995 C1 and DE 41 28 675 C2. Packings of this type are sold under the DURAPACK(copyright) brand name, with their structure and application disclosed in brochures for such DURAPACK(copyright) packings.
Packing materials other than glass have also been described in German Pat. No. DE 41 28 675 C2.
Packing elements of such structured mass transfer packings are arranged on top of one another in packing bed zones in the column cross section. An excessive overall own weight can be prevented by using ring supports or carrier frames. Hereby, it is strived to fill the column cross sections as completely as possible. The formation of more or less distinct marginal gaps is, however, unavoidable between the periphery of the packing elements and the column wall, so that the liquid phase will partially flow downwards along the inner wall surface of the column during the mass transfer process in the column and thus does not sufficiently participate in the mass transfer. This maldistribution, also called wall effect, deteriorates the desired mass transfer efficiency of the column. The marginal gaps may hereby have very different widths ranging from fractions of millimeters up to several millimeters.
One approach to counter the wall effect involves the provision of distribution elements or baffles, also called wall wipers which are typically implemented as upwardly expanding conical or funnel-shaped rings which extend directly up to the inner wall of the column and return the liquid phase fraction flowing downwardly along the inner column wall back to the interior of the packing elements.
Attachment of such baffles to the packings is implemented to date predominantly through placement of preformed, annular baffles of PTFE or other appropriate plastic between the packing elements. This approach suffers shortcomings because the rings, disposed between the individual packing elements, reduce the flow cross section between the packing elements at these locations and tensions are typically experienced during installation of the packing elements, resulting in a distortion of the annular baffle and thus the slanted edge thereof to assume a corrugated configuration, so that the baffle is prevented from evenly abutting against the inner column wall. As a consequence, the gap sealing action is poor and liquid can collect in formed valleys of the baffle.
Another approach involves the attachment of baffles to the packings in such a way that a slanted film of PTFE or other plastic is secured to the edge of the packing by sewing a vertical portion of the film with a second PTFE ring or plastic ring wrapped about the packing. This approach is, however, cumbersome.
It would therefore be desirable and advantageous to provide an improved mass transfer packing which obviates prior art shortcomings and allows easy attachment of baffles while still attaining a reliable operation of the baffles.
According to one aspect of the present invention, a packing for a mass transfer column, includes a packing bed having a packing element; a tubular shrink film shrunk onto at least a portion of a periphery of the packing element; and a baffle arranged at the periphery of the packing element to prevent a wall effect in the column, with the baffle having a conical edge which extends outwardly with respect to the shrink film and is made of plastic.
The packing element of the packing bed may be a single-piece packing element or a packing element comprised of individual segments that are combined to form the packing element.
According to another feature of the present invention, the conical edge of the baffle may be formed by at least a part of an upper rim of the shrink film. As an alternative, the baffle is made by a film strip placed onto the packing element and having an upper rim bent at least in part outwards with respect to the periphery of the packing element to form the conical edge, wherein the film strip is held in abutment with the packing element by the shrink film. The bent part of the upper rim of the shrink film may be slitted. Likewise, the bent part of the upper rim of the film strip may be slitted.
According to another feature of the present invention, the film strip may be a strip of a heat-resistant plastic film, e.g., of polytetrafluoroethylene (PTFE film). The film strip may also be a strip of an extruded film band with the upper rim being angled. The angled upper rim of the film band and a remaining portion of the film band define an angle between 110 and 160xc2x0, preferably 135xc2x0, wherein the angled upper rim defines about 20 to about 40% of an overall width of the film band.
According to another feature of the present invention, the shrink film may be made of tetrafluoroethylene copolymer, e.g., of fluorinated ethylene propylene (FEP).
According to another aspect of the present invention, a film band of PTFE having an angled upper edge bent at an angle of about 135xc2x0, is used for making a baffle for a packing of a mass transfer column.